The invention is based on a priority application EP 03360017.2 which is hereby incorporated by reference.
The invention relates to a digital to analog converter, a phase control circuit comprising a phase detector, a control-circuit filter and a voltage-controlled oscillator, a transmission unit, in particular at the junction between electrical signals and optical signals and a recognition circuit for recognizing the locking-in of a phase control circuit.
The recognition circuit according to the invention has resulted from the original example of the development of a phase control circuit for a transmission unit and is explained below proceeding from the description of said phase control circuit.
In the transmission of data signals in digital form, there are a larger or lesser number of signal distortions. Whenever a noticeable distortion is to be expected, an attempt is made to restore the original signal with the aid of signal regenerators. In the case of digital signals, this is simpler than in the case of analog signals insofar as here only a few, in the case of binary data only two, signal states can occur at all. In addition, transitions between two signal states are permissible only at very definite time instants and, consequently, only to be expected in the vicinity of said time instants. It is then important in this connection that the greatest probability of finding the correct signal state exists in the center between two time instants. The distance between two such time instants is the clock pulse that underlies the transmission.
If it is desired to view a still unregenerate signal on an oscillograph or to examine a corresponding printout of a simulation, the time base (x-axis) is expediently chosen in such a way that the signal (y-axis) starts again from the beginning after one or more whole clock pulses. The display then has the appearance of an eye or eyes lying immediately alongside one another. Mention is therefore often made of an “eye diagram”, an “eye”, the “eye opening” or the like. These expressions are also used even if the continuous signal trace itself is examined.
Such a signal is therefore regenerated in such a way that the actual signal value is always determined in the center of the eye, where the eye opening is greatest and the next permissible data value is outputted.
For this purpose, phase control circuits of the type mentioned at the outset are standard. In the case underlying the invention, dynamic fluctuations are due to the fact that the input signal originates from data packets that do not follow one another with exactly the same intervals. Long-term changes are due to the variation in operating parameters that may also originate from ageing of the affected parts. The compensation for the long-term changes causes phase fluctuations, a so-called “dithering”. Said dithering can be minimized if the control-circuit filter has as low an upper cutoff frequency as possible.
For the sake of completeness, it should be pointed out here that such a phase control circuit also adjusts the frequency. A clock pulse that is adjusted in such a way that the phase is correct at all the consecutive points of the some phase also has the correct frequency.
In the original example, the working range of the phase control circuit is, in addition, very large in comparison to the center frequency. Here, it is no longer possible to employ only one oscillator; on the contrary, a plurality, in the original example three, oscillators are provided, of which the most suitable is used in each case. The latter must then be driven in its entire working range as precisely as possible with a suitable control voltage. Furthermore, it is necessary to determine whether the phase control circuit has locked in or not.